Process of making composite metal articles



Patented Jan. 11, 1949 PROCESS OF MAKING COMPOSITE METAL ARTICLES Hugh G. Webster, Detroit, Mich., assignor to J. H. Shoemaker N Drawing. Application January 23, 1943, Serial No. 473,397

This invention relates to a process of coating metal surfaces with a high-lead-content alloy and more particularly to the coating of the sur faces of ferrous metals and other alkali-resistant metals or alloys thereof with such lead alloys.

In the past, it has been known that lead-tin alloys can be coated onto metal surfaces, but it has been difficult to get uniform coatings on metal articles and in fact practically impossible to get coatings that were free of pinholes and un'wetted portions. Furthermore, those coatings which have been obtained have used high-tin-contentlead alloys. These areexpensive and for that reason the use of such alloys has been limited.

An object of my invention is to provide a process for producing strongly adherent commercially perfect coatings of high-lead-content alloys on alkali-resistant metals, particularly ferrous metals and thereby to obtain coated or composite products of certain types which are superior to, any comparable products of prior art.

Another object is to provide simple and economical meth'odallowing the use of low-tin-coutent lead alloys for the protection of coatings on ferrous metals.

Still another object is to provide a process whereby alkali-resistant metals may be coated with a lead alloy containing as low as 5 per cent tin.

My invention has for further objects such additional operative advantages and improvements as may be found to obtain hereinbelow.

I have found that commercially valuable composite metal products containing an a1kali-resistant base with a coating of a high-lead-coritent alloy may be obtained by a process wherein the base metal is first treated in a molten bath of certain alkali-metal salts, then is removed from the bath and treated with dilute mineral acid and finally is provided with a film or layer of a highlead-content alloy by dipping the prepared base metal in a molten bath of the alloy or by coating the molten alloy onto the prepared base, and finally allowing the product to cool.

The molten bath comprises a mixture of an alkali-metal hydroxide and an alkali-metal nitrate and preferably also contains a small amount of an alkali-metal chloride. Preferably the molten-salt etching bath should contain a relatively large amount of the alkali-metal hydroxide, that is to say from 1.5 to 3.0 parts by weight, per 1 part by weight of alkali-metal nitrate. In many instances, the bath may consist of these ingredients alone, but I have found that the activity of such a bath may be increased, and the time of 1 Claim. (01. 1l751) treatment shortened, by including a relatively small amount of an alkali-metal chloride, as for example, from 0.1 to 0.5 part by Weight, per 1 part by weight of the nitrate. The chloride seems to have a catalytic effect on the activity of the bath. Furthermore, the temperature increasing effect of carbon dioxide on the bath, due to formation of carbonates, is minimized when chlorides are present.

An advantageous mixture, which I have found useful for many applications, may be prepared by mixing 2 parts by weight of sodium hydroxide, 1 part by weight of sodium nitrate, and from 0.1 to 0.5 part by Weight of sodium chloride. Such a composition, containing about 0.35 part of sodium chloride, has a melting point of approximately 500 R, a heat of fusion of 49.6 cal/gm. and a specific heat of 0.230 cal./gm. C. In use, the mixture is placed in an ordinary iron pot or other alkali-resistant container, and heated in any suitable manner until melted. When the composition has been melted and the tempera ture of the bath is within the operating range, the .articles to be coated are immersed in the bath for a predetermined period of time, usually from two to five minutes, after which they are withdrawn and dipped in water or sprayed to remove entrained salts.

Compounds (hydroxides, nitrates, and chlorides) of any of the alkali metals, such as potassium, sodium or lithium may be used in the cleaning bath, but usually the sodium compounds are employed, because of their relatively low cost.

The time required for the treatment in the molten-salt bath varies with the composition, surface condition, amount of carbon, graphite and compounds of sulphur, silicon and the like and the size and Weight of the metal pieces to be treated, but it will ordinarily not be less than 15 seconds nor greater than 30 minutes. The pieces oftenneed to be left only long enough in the bath to bring them to the temperature of the bath.

. The temperature of treatment is that necessary to maintain the bath in a molten condition, but may be considerably higher without harm. By Wayof example, the specific mixture referred to above works best at temperatures of 550 to 900 F. In some instances, it may be desirable to effect phase changes in the metal during treatment. For example, heat treatment of the metal may be carried out simultaneously with the immersion in the molten-bath and the cooling of the metal may be rapid or slow, depending on the desiredresult. l The baths may be used for extended periods of time without material deterioration. They are suitable for use with all the common commercial alkali-resistant metals, such as cast iron, and other ferrous metals and alloys, brass, copper and copper alloys, and the like to prepare them for coating with lead-tin alloys and especially the low-tin, low-antimony lead alloy noted above. They are generally not suitable for treating zinc, aluminum or magnesium, that is to say, metals and alloys readily attacked by alkalies.

It appears that the use of a moltenesalt bath containing alkali-metal nitrate in the first step in my process has a critical effect on the results to be obtained. I have been unable to obtain comparable results without such an oxidizing agent in the salt bath.

Treatment in the molten-bath leaves a dark oxidized coating on the surface of the metal. The oxide may vary from light yellow to brown. The non-metal substances of sulphur, silicon and the like are oxidized. The oxides formed may be readily removed in a simple manner by brief immersion in a dilute mineral acid such as dilute hydrochloric orsulphuric acid. The acid should be of such character that it will not tend to leave a residue on the metal surface. However,-dilute hydrochloric or sulphuric acid quickly removes the black oxide coating.

Hydrogen, which is usually adsorbed by the metal in acid-pickling operations and prevents firm bonding of the alloy, is not present in this treatment because dilute acid solutions are used and the period of immersion is brief.

The salt bath and acid treatments together convert the exposed surfaces of the metal pieces into etched matrices having minute foramina or pockets materially depleted of oxidized and non-metal. substances, and the etched matrices thus ob tained have been found to form an exceedingly strong flawless bond with high-lead alloys when the alloy is suitably applied thereto in amolten condition, such as by hot dipping. By oxidizable and non-metal substances I mean substances on and at thesurface of the metal pieces which are notthe elemental metal or alloy thereof, for example, such substances as graphite, carbon and compounds of sulphur, silicon, phosphorous, et cetera.

The resulting coated or composite products are free from pinholes and porosity such as might provide foci for corrosion, and the bonded coating or layer can hardly be stripped from the base metal under any conditions, but appears actually to be united therewith through an interfacial alloy.

The lead alloy may be melted in an ordinary black iron pot and the article coated therein by a clipping process. However, other methods of applying the alloy to the surface of the metal may obviously be used.

Usually I prefer to flux the metal before treatment with the molten-lead alloy. Zinc ammonium chloride is an example of a suitable fiuxing agent and other such agents may also be used.

Any of the commonly used lead-tin alloys may be coated onto metal surfaces with my process. In some cases I have been able to coat metals with pure lead by the process, but I prefer to use the lead-tin alloys for more uniform coatings are obtained with them. Generally the higher the amount of tin in the alloy the betterthe coating that is obtained.

However, to reduce the amount oftinin the alloy, I prefer to use a composition containing about per cent lead, about 5 per cent'tin, and

4 about 5 per cent antimony. This composition is cheaper than the ones containing more tin, but it gives as good a bond and as resistant a coating as the more expensive alloys. The tin and antimony should be used in approximately equivalent proportions. They appear to form a solid solution in the lead. About ten per cent combined tin and antimony is the optimum composition.

I am able to coat cast iron by the process with a pore-free even coat of a lead alloy containing only about 5 per cent tin, and as far as I can determine my process is the first satisfactory process for accomplishing this result.

In the case of copper and copper-containing :alloys such as bronze and brass, I prefer to use lead alloys substantially free of antimony, since such alloys appear to form brittle compounds with copper at the interface which reduces the strength of the bond.

By way of specific example of my invention, various metal articles were immersed in a hot molten bath consisting of 1 part by weight sodium nitrate, 2 parts by weight sodium hydroxide. and 0.25 part by weight sodium chloride, and held in the bath at approximately 800 F. for five minutes. When removed and rinsed, the surfaces of the metal parts exhibited a dull, dark finish. After immersion of the treated parts in a bath containing between 8' and 10 per cent HCl for fifteen seconds the surface of the metal pieces consisted of etched matrices having a bright but matted or roughened structure substantially depleted of non-metal substances. The surface had minute anchorages or crevices where oxidizable substances including carbon and compounds of sulphur, silicon, et. cetera were removed from the metal. These parts were then dipped into a molten-lead alloy containing about 90 per cent lead, about 5 per cent tin and about 5 per cent antimony. The lead alloy flowed into the crevices formed by the etching process and gave a firm bond. After removal from the dipping bath, the specimens were tested for pores, using the sensitive ferroxyl reagent described by Cushman and Gardner in ,The Corrosion of Iron and Steel,. (1910) on page 50. The specimens were free of pores according to this test, showing no blue color.

In another specific example of my invention, I applied my process to certain cast-iron parts. These partspresented a difficult problem as their surfaces held considerable amounts of adherent or imbedded silica. As far as I am aware, no prior methods are fully satisfactory for lead coating of such material. Prolonged immersion of like specimens in hydrofluoric acid, followed by rinsing, drying and hot-lead alloy dipping had failed to give satisfactory coatings. The coatings produced had pinholes (as shown by the ferroxyl test) and pores of such nature as to provide foci for corrosion or oxidation and failed to give surfaces suitable for hot-lead dipping. It was impossible to obtain lead coatings free of pinholes, (as shown by the ferroxyl test) or free of pores of such character as to form fool for corrosion or oxidation.

I first immersed these cast iron parts in 20 per cent H2804 for from three to five minutes, at about 180 F. This preliminary pickling is not usually necessary, but helps in aggravated cases to somewhat loosen the silica and to render itmore easily removable. However,neither this pre-treatment nor prolonged pickling proved adequate for the purpose stated.

The parts were then removed from the acid bath, rinsed and dried, and then immersed in a molten bath of the same composition as that used in the preceding example. They were maintained in the molten bath at approximately 800 F. for about five minutes, then removed, rinsed, immersed in 20 per cent HCl for about one minute, and then rinsed and dried. The specimens were fiuxed in zinc ammonium chloride and then the specimen parts, so treated, were dipped in a molten-lead alloy composed of about 90 per cent lead, about per cent tin, and about 5 per cent antimony, thereby acquiring a uniform lead alloy coating free of pinholes, having neither rivul-ets nor dewetting and being otherwise satisfactory in character.

This application is in part a continuation of a copending application, Serial No. 431,168 filed February 16, 1942 and now forfeited. In that application an improved process of cleaning organic and inorganic impurities from the surface of alkali-resistant metals is described. The treatment in a molten-salt bath and acid immersion as described in that application is adaptable to etch and prepare metal surfaces for coating metals with lead alloys. Accordingly, the present application is directed to a process for preparing the surface of alkali-resistant metals for hot-lead coating operations by an etching process. My present process is usually used after dirt or foreign matter has been removed from the surface of the metal, for which purpose I may employ various prior art processes or the process described in said application Serial No. 431,168.

While I have described my invention hereinabove with respect to various specific examples and operating details, it will be obvious to those skilled in the art that my invention is not limited to such details and examples, but may be variously practiced and embodied within the scope of the claim hereinafter made.

What I claim is:

A three step process for coating the surfaces of alkali-resistant metals with a molten lead-tin alloy, which comprises preparing the metal for coating by cleaning it with a two step nonelectrolytic process wherein each step is in the nature of a chemical reaction, and wherein the second step leaves no added finish to the surface,

and wherein the first step comprises immersing the metal in a bath comprising a molten mixture of 1 part by weight of alkali metal nitrate and from 1.5 to 3.0 parts by weight of alkali metal hydroxide, and from 0.1 to 0.5 parts by weight of alkali metal chloride, for a period of time sufiicient to remove any organic surface impurities and convert inorganic surface impurities to an oxidized coating, more readily soluble than were the original inorganic impurities, the operating temperature of the first bath being above the melting point of the molten mixture, that is to say, being in the range of 550 to 650 F., and wherein the second step comprises immersing the metal in a bath of such nature that when the metal is immersed therein for a proper period of time, the bath will react chemically with the oxidized coating to remove it, so as to leave the surface free of the original impurities and of the oxidized coating as well, the second step, however, being such as not to expose at the surface other impurities, not previously existent, or not previously exposed, thus leaving the surface free of all impurities and consisting solely of the metal, and finally coating the surface thus cleaned, with the coating, the bath of the second step being a dilute acid selected from the class consisting of hydrochloric and sulfuric acids, and the immersion being sufficiently brief as not to permit the acid to attack the metal, but merely to remove the oxidized coating formed by the first step.

HUGH G. WEBSTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 808,911 Eaches Jan. 2, 1906 1,276,978 Shoemaker Aug. 27, 1918 1,520,732 Thompson Dec. 30, 1924 2,032,256 Canfield Feb. 25, 1936 2,206,597 Canfield July 2, 1940 2,244,526 MacKay June 3, 1941 2,304,069 Beckwith Dec. 8, 1942 

